A bridge-type machine tool is conventionally known as a typical five-axis machine tool. A bridge-type machine tool includes a spindle head provided on a cross rail and has, in addition to X-axis, Y-axis and Z-axis, an A-axis for pivoting of the spindle head and a C-axis for indexing of a table. An example of such a bridge-type machine tool is disclosed in Japanese Patent Laid-Open Publication No. 2004-34168. Five-axis machine tools, including the bridge-type machine tool, have been advantageously used for machining on a free-form surface, as typified by machining of a propeller.
In value-added machining of a mold, for example, shaping machining on a free-form surface has been the highest priority, and high-speed rotation of a spindle and high-speed following in axial movement have been required. To meet the requirements, higher-speed and higher-precision shaping machining with a five-axis machine tool has become realized.
These days, the environment surrounding manufacturing industry is changing greatly, and there is an increasingly stricter demand for shortening manufacturing time for machined products. There is a also a stronger demand by users for a five-axis machine tools that can better perform process-intensive combined machining. Such demands have led to a significant improvement in high-speed, high-precision machining, as described above. On the other hand, old-fashioned machining operations are still practiced in parallel, and the imbalance is becoming a problem.
For example, in machining of a mold for molding a large-sized resin product, such as an instrumental panel or a bumper of an automobile, besides advanced shaping machining, there are many machining operations for which advanced shaping machining is not necessarily required, such as machining of a hole for insertion of an extrusion pin, machining of a cooling cavity, undercut-shaping machining, etc.
Even today when high-speed machining is well-established, machining operations which are in no way high-speed and high-precision machining, such as machining of an extrusion pin hole, are currently practiced in a labor-intensive manner by skilled workers. This is because a number of extrusion pin holes are provided in a mold, and that the respective pin holes differ in the inclination and the direction.
To machine an extrusion pin hole with a five-axis machine tool, it is necessary to pivot the spindle to meet the inclination of the hole and to rotate the table to meet the direction of the hole. However, the machining inevitably involves an error in the tilt angle of the spindle head due to the weight of the spindle head, or a mechanical error in the rotation angle of the table. Conventional five-axis machine tools are thus not suited for machining of extrusion pin holes. Accordingly, a labor-intensive oblique drilling operation by a skilled worker is practiced separately from shaping machining with a five-axis machine tool.
In practice, for oblique drilling of extrusion pin holes, a skilled worker manually performs setup and drilling for each of the holes. Most of the operation time is spend on the setup work though only a short time is needed for actual drilling operation. Thus, the high-speed and high-precision performance of current machine tools is not fully utilized at present.